1. Field of the Invention
The present invention relates to a toner for electrostatic charge image developing which is suitably used in image formation by electrophotography, as well as a developer for electrostatic charge image developing, and an image forming apparatus, using the toner for electrostatic charge image developing.
2. Description of the Related Art
Conventionally, when an image is formed in a copying machine or a laser beam printer, electrophotography is generally used. As the developer-used in electrophotography, a two-component developer containing a toner and a carrier, and a one-component developer containing a magnetic toner or a non-magnetic toner, are known. The toner used in these developers is usually prepared by a kneading grinding method.
This kneading grinding method is a method of melting and kneading a thermoplastic resin with a pigment, a charge controlling agent, and a releasing agent such as wax, finely-dividing and classifying this melt kneaded material after cooling to obtain desired toner particles. If necessary, inorganic and/or organic fine particles are further added to the surface of toner particles prepared by the kneading grinding method for the purpose of improving flowability and cleanability.
According to an image forming method using electrophotography, an electrostatic latent image formed on a photoreceptor by an optical means is developed in a developing step, transferred onto a recording medium such as a recording paper in a transferring step, and fixed onto a recording medium such as a recording paper generally by heat and pressure, to obtain an image.
In recent years, the development of electrophotography technique from black and white to full color has progressed rapidly. Color image formation using full color electrophotography generally reproduces all colors using four colors; namely, the three primary colors of yellow, magenta and cyan plus black.
In general full color electrophotography, a manuscript is first color-separated into yellow, magenta, cyan and black, and an electrostatic latent image of each color is formed on a photoconductive layer.
Then, a toner is retained on a recording medium via a developing step and a transferring step. Then, the aforementioned steps are successively performed a plurality of times, and a toner is overlaid on the same recording medium while being positioned.
Then, a full color image is obtained by a one time fixing step. For a color toner used in full color electrophotography, it is required that a multicolor toner is sufficiently mixed at the fixing step. Sufficient mixing improves color reproducibility and transparency of an OHP image, and a full color image having high image quality can be obtained. In order to enhance color mixing property, it is generally desired that a color toner is formed from a low-molecular resin which is sharply melted.
Meanwhile, recently, power consumption saving and higher image quality have also come to be demanded in electrophotography. As one strategy for saving power consumption in electrophotography, fixation at a lower temperature is sought for the purpose of decreasing the amount of energy used when operating a machine.
In order to respond to such a need, new approaches have been adopted on both the toner side and the apparatus side.
As an approach on the toner side, various attempts have been made to lower the fixing temperature of a toner. For example, a method of controlling the viscoelasticity of a toner (see Japanese Patent Application Laid-Open (JP-A) No. 9-325520, and JP-A No.8-234480), and a toner using a crystalline resin as a binder resin have been proposed (Japanese Patent Application Publication (JP-B) No. 4-24702). In addition, in recent years, many toners having a core shell structure consisting of a core layer, and a shell layer covering this core layer have been proposed (for example, see JP-A No. 10-123748).
Among them, in particular, a toner having a core shell structure is a most useful technique in that it is easy to realize not only low temperature fixability, but also other properties in a better balance.
Meanwhile, as an approach on the apparatus side, an apparatus having the functions of decreasing the amount of electric power supplied to a fixing machine in a prolonged state when no image is formed (standby time), and maintaining the temperature of a heating means such as a heating roll at a temperature lower than the temperature at fixing (hereinafter, also referred to as “standing time power saving function”), in order to reduce consumed energy during standby, has been adopted.
In an apparatus having such functions, since it is necessary to secure not only power consumption saving but also convenience, it is preferable to adopt as a fixing machine one having a smaller heat capacity. This is because when an apparatus is used in a state where the amount of power supplied to a heating means of a fixing machine has decreased, and the temperature of the heating means is lower than the temperature necessary for fixing, the temperature of the heating means is instantly elevated to the temperature necessary for fixing at the same time as the electric power is turned on, from the viewpoint of convenience.
In an image forming apparatus having such a standing time power saving function, in the standby state, the temperature of the heating means of the fixing machine is maintained at a temperature lower than the temperature during fixing, in order to suppress the amount of power consumption. For this reason, when one tries to form an image from the standby state, electric power is supplied at once in order to instantly raise the heating means to a temperature at which fixing is possible, and a phenomenon whereby an apparatus is heated to a temperature higher than a prescribed set temperature (over shoot) occurs temporarily. Thereupon, when paper is supplied to the fixing machine for image formation, since heat is absorbed by the paper passed through the fixing machine, the temperature of the fixing machine is lowered from the over shoot state.
In addition, in addition to the aforementioned over shoot immediately after initiation of image formation (hereinafter, also referred to as “initial over shoot”), periodic over shoot also occurs even when an image is continuously formed, since lowering of temperature due to supplied paper and, when the temperature is lower than the prescribed temperature, elevation of temperature due to heating are repeated (hereinafter, referred to as “steady over shoot”).
When an image is formed, occurrence of such over shoot cannot be avoided. For this reason, there is a deviation in an actual fixing temperature in each sheet, resulting in both paper which is fixed at a temperature higher than the set temperature and paper which is fixed at a temperature lower than the set temperature. Such remarkable deviation in temperature results in unevenness of image quality. Therefore, a fixing machine built into an image forming apparatus is designed so that temperature deviation during image formation is within a prescribed range, so as not to cause unevenness of image quality.
However, when images are continuously formed from the standby state with an image forming apparatus having a standby time power saving function using a toner having a core shell structure excellent in low temperature fixability, in some cases the tone of a formed image varies from sheet to sheet, and this is particularly pronounced in multiple color images using 2 or 3.